gtkentry: set minimum-width to 0 instead of 150

gtkentry: set minimum-width to 0 instead of 150

and use 150 as natural-width.

Currently there's no way for a GtkEntry to be less
than 150px wide (apart from using "width-chars" property),
this is too much for a default minimum-width, an app
developer may need to have a shorter GtkEntry, for example
when the UI it's been shrunk by the user (see [1]) or when
you want to match the size of another widget (which is less
than 150px) see [2] for Evince bug on using
gtk_combo_box_new_with_model_and_entry() for PDF forms where
GtkEntry of ComboBox is too wide and doesn't match the combo
list width.

Using "width-chars" property may be a workaround to obtain
a short minimum-width for the entry, but is not a proper
solution for the mentioned cases as you may not know how
short your GtkEntry will be, or the fact that using "chars"
as a width unit is not pixel accurate.

Curious note: the commit that introduced the GtkEntry
minimum-width to be 150px is from 20 years ago, see
https://bit.ly/2ySEfK4

[1] This change was already suggested by Benjamin Otte
in a blog comment https://bit.ly/2J96wRo

[2] Fixes issue evince#1002